A lubricating oil, in particular, an engine oil needs to be managed to be in a normal state because degradation thereof leads to decreases in operating efficiency, durability, etc. of such devices as an engine which use the oil.
For this reason, there is a demand for a device or a method which enables the degree of degradation to be measured in a simple manner, but there is no such device or method. Hence, such management has been performed that a time period of use is used as a measure of the degree of degradation, or a determination has been made as to the degree of degradation based on many property values acquired through analysis of the lubricating oil.
For example, there are proposed many methods in which the degree of degradation is evaluated by taking into account a dielectric constant and a time period of use of the lubricating oil, the degree of heavy use, etc. However, none of those methods provides measurement of the degree of degradation itself for the lubricating oil. Those methods are established based on the premise that there is no highly-reliable method for measurement of the degree of degradation. Those methods are low in reliability because results thereof are expected to vary considerably depending on the type and the use condition of the lubricating oil.
On the other hand, there is proposed a method in which, by assuming that degradation of the lubricating oil is represented by ionization change of an alkaline earth metal, in particular, calcium, contained in the lubricating oil, which is caused by the water content in the oil, an ISFET is employed to measure the degree of the ionization based on a potential difference between a reference electrode and the ISFET (see Patent Document 1). However, in a non-aqueous lubricating oil, metal ions are not able to exist in a stable manner, and hence it is impossible to perform accurate measurement for an oil with little water content.    [Patent Document 1] JP 63-40855 A